In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. User equipment units (UEs) may be, for example, mobile telephones (“cellular” telephones), desktop computers, laptop computers, and tablet computers, or stationary units, with wireless communication capability to communicate voice and/or data with a radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called for example “NodeB” or (in Long Term Evolution (LTE)) eNodeB (eNB). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the UEs within range of the base stations.
Specifications for an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are on-going within the 3rd Generation Partnership Project (3GPP). Another name used for E-UTRAN is the Long Term Evolution (LTE) Radio Access Network (RAN). LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller node are performed by the radio base stations nodes. As such, the radio access network of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller nodes.
The LTE standard is based on multi-carrier based radio access schemes, Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) in the uplink. Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. TD-LTE (time division LTE) is a variant of LTE where a single channel is used for transmissions in both directions, uplink and downlink, but divided in time.
GSM (Global System for Mobile communication) is an earlier well known system for mobile communication. The GSM originally used one dedicated frequency band for uplink communication from the user equipment, often called mobile device, to the radio access network and one dedicated frequency band for downlink communication from the radio access network to the mobile device. Such mobile devices where not adapted for direct communication from device-to-device. Mobile devices could, in addition to their GSM communication ability, be equipped with Bluetooth transceivers for close range communication from device-to-device. The user could then decide whether to call another mobile device (or UE) using the GSM cellular network, in which case communication where established via a base station of the GSM network, or to establish a direct communication using Bluetooth.
In later cellular radio access network systems, such as those of the 3rd generation, the available frequency spectra can be more freely employed and base stations and user equipment have been provided with transceivers that are able to transmit and receive in the same frequency band. This has made it possible to communicate using the same transceiver and frequency bands, from one user equipment to another, either in a two-hop fashion via the radio access network or, alternatively directly from device-to-device in a one-hop fashion. Direct communication in the same frequency band can be done also in GSM, but the mobile units was not prepared for it from the beginning.
A direct communication link between two UEs may be advantageous in lessen the traffic load of the radio access network. It would also be advantageous if the users need not have any special skill, or perform any specific operations, for entering a direct communication. Such direct communication may make radio resources usually employed between the radio access network and the UEs available for other communications. At close range direct communication D2D may be made at low power consumption and large capacity in relation to communication via the radio access network. However, establishing direct communication requires resources, and failure in the process of establishing direct communication need to be avoided so as not to counteract an overall purpose of economizing in radio resources.